This invention relates to signal measurement. In particular it relates to a method and apparatus for measuring the noise present in a television or video signal.
It is often required to measure the r.m.s. level of noise present in a television signal. The television signal comprises a number of lines which are scanned in a raster pattern to obtain a field, two fields being interlaced to form a complete picture. The picture is refreshed for instance 50 times per second. Noise still generally be present on each of the lines forming the raster. Generally, in a television broadcast, the picture signal will be transmitted on most of the lines in a field, and information such as test signals or teletext information may be transmitted on some of the remaining lines. One or more lines are usually left spare, however. It is desirable to measure the noise level on part or the whole of a spare line since a true value for the amount of noise will be obtained unaffected by broadcast signals. In a typical 625 line system, the time taken to scan one line is approximately 64 microsecond. This period represents the maximum possible time during which noise measuring apparatus must function to obtain measurement of the noise level, although the measurement may be repeated at the same point in every scan, i.e, every fiftieth of a second. Noise measuring apparatus must therefore be gated to only take measurements at an appropriate time and must have fast reaction times. The noise which it is desired to measure lies within a frequency band from say 200kHz to around 8 MHz, therefore the measuring apparatus should have a wide frequency response.
It is known to directly measure noise levels by using a high speed analogue to digital converter and a microprocessor to calculate r.m.s. level. Such apparatus is difficult to implement and is limited in the frequency and dynamic ranges it can cope with.
A more practical approach is to use an automatic gain control, AGC, loop to set the level of noise to a constant level. The gain of the loop is initially unknown and is measured by feeding a signal of known r.m.s. level into the loop and subsequently sampling to find the change in amplitude of the known signal. Such systems are prone to error and measurements may need to be taken over several fields of the video signal to obtain an accurate result.